The present invention relates to a method and system for liquefying a natural gas feed stream to produce a liquefied natural gas (LNG) product.
The liquefaction of natural gas is a highly important industrial process. The worldwide production capacity for LNG is more than 300 MTPA, and a variety of refrigeration cycles for liquefying natural gas have been successfully developed, and are known and widely used in the art.
Some cycles utilize a vaporized or vaporizing refrigerant to provide the cooling duty for liquefying the natural gas. In these cycles, the initially gaseous, warm refrigerant (which may, for example, be a pure, single component refrigerant, or a mixed refrigerant) is compressed, cooled and liquefied to provide a liquid refrigerant. This liquid refrigerant is then expanded so as to produce a cold vaporized or vaporizing refrigerant that is used to liquefy the natural gas via indirect heat exchange between the refrigerant and natural gas. The resulting warmed vaporized refrigerant can then be compressed to start again the cycle. Exemplary cycles of this type that are known and used in the art include the single mixed refrigerant (SMR) cycle, cascade cycle, dual mixed refrigerant (DMR) cycle, and propane pre-cooled mixed refrigeration (C3MR) cycle.
Other cycles utilize a gaseous expansion cycle to provide the cooling duty for liquefying the natural gas. In these cycles, the gaseous warm refrigerant is compressed and cooled to form a compressed refrigerant. The compressed refrigerant is then expanded to further cool the refrigerant, resulting in an expanded cold refrigerant that is then used to liquefy the natural gas via indirect heat exchange between the refrigerant and natural gas. The resulting warmed expanded refrigerant can then be compressed to start again the cycle. An exemplary cycle of this type that is known and used in the art is the nitrogen expander cycle.
Further discussion of the established nitrogen expander cycle, cascade, SMR and C3MR processes and their use in liquefying natural gas can, for example, be found in “Selecting a suitable process”, by J. C. Bronfenbrenner, M. Pillarella, and J. Solomon, Review the process technology options available for the liquefaction of natural gas, summer 09, LNGINDUSTRY.COM
At present, all the plants for liquefying natural gas that have so far been constructed are built on land. An important trend for further growth in the LNG industry is to develop remote offshore gas fields, which will require a system for liquefying natural gas to be built on a floating platform. Designing and operating such a LNG plant on a floating platform poses, however, a number of challenges that need to be overcome. Motion on the floating platform is one of the main challenges. Conventional liquefaction processes that use mixed refrigerant (MR) involve two-phase flow at certain points of the refrigeration cycle, which may lead to reduced performance due to liquid-vapor maldistribution if employed on a floating platform. In addition, in any of the refrigeration cycles that employ a liquefied refrigerant, liquid sloshing will cause additional mechanical stresses.
Storage of an inventory of flammable components is another concern for many LNG plants that employ refrigeration cycles such as the SMR, cascade, DMR or C3MR processes, either because of the unavailability of such components, or because of safety considerations, such as would in particular be the case for a Floating LNG (FLNG) platform.
As a result, there is an increasing need for the development of a process for liquefying natural gas that involves minimal two-phase flow and requires a minimal flammable refrigerant inventory.
The nitrogen recycle expander process is, as noted above, a well-known process that uses gaseous nitrogen as refrigerant. This process eliminates the usage of mixed refrigerant, and hence it represents an attractive alternative for FLNG facilities and for land-based LNG facilities which require minimum hydrocarbon inventory. However, the nitrogen recycle expander process has a relatively lower efficiency and involves larger heat exchangers, compressors, expanders and pipe sizes. In addition, the process depends on the availability of relatively large quantities of pure nitrogen.
U.S. Pat. No. 8,656,733 teaches a liquefaction method and system in which a closed-loop gaseous expander cycle, using for example gaseous nitrogen as the refrigerant, is used to liquefy and sub-cool a feed stream, such as for example a natural gas feed stream. In the embodiment depicted in FIG. 5 of said document, the sub-cooled LNG product may be throttled using a valve or expanded in a hydraulic turbine so as to partially vaporize the stream, and the resulting flash gas may be cold compressed and warmed against the refrigerant in the refrigerant heat exchangers, or may be warmed in the sub-cooler heat exchanger against the LNG stream.
U.S. Pat. No. 6,412,302 teaches a process for producing LNG that uses dual gaseous expander cycles to cool, liquefy and sub-cool a natural gas stream. One expander cycle uses gaseous methane, ethane, or treated natural gas as the refrigerant, and the other expander cycle uses gaseous nitrogen. The LNG product may be expanded in a liquid expander, then treated in an N2 stripper, in order to provide a treated LNG stream.
U.S. Pat. No. 6,658,890 teaches a system and a method for liquefying natural gas in which a cascade cycle comprising a closed loop propane circuit, closed loop ethylene circuit, and open loop methane circuit are used to cool, liquefy and sub-cool a natural gas feed stream. The natural gas is cooled against the vaporizing propane refrigerant, and liquefied by heat exchange with the vaporizing ethylene refrigerant. The resulting LNG stream is then subcooled in a sub-cooler heat exchanger and further cooled by flashing the sub-cooled LNG stream in two consecutive end-flash stages, thereby providing two methane flash gas streams that are used as refrigerant in the sub-cooler heat exchanger. The LNG stream from the second end-flash stage is further sub-cooled in the sub-cooler heat exchanger, and then divided in a splitter to provide the LNG product stream and a liquid methane stream that is expanded and also returned to the sub-cooler heat exchanger as refrigerant. The warmed methane refrigerant streams exiting the sub-cooler heat exchanger are compressed and recycled to the natural gas feed stream.
U.S. Pat. No. 7,234,321 teaches a process for liquefying natural gas, in which the natural gas feed stream is pre-cooled in a series of pre-cooler heat exchangers against a vaporized mixed-refrigerant, and is then partially liquefied by being expanded in a liquefying expander. The partially liquefied natural gas stream is then separated to provide an LNG stream and a methane vapor stream, the vapor stream being returned to and warmed in the pre-cooler heat exchangers before being compressed and recycled to the natural gas feed stream. The LNG stream may be throttled and further separated to provide the LNG product, and a further methane vapor stream that is also returned to and warmed in the pre-cooler heat exchangers to provide a warmed fuel gas.
US 2014/0083132 teaches a similar process to that taught in U.S. Pat. No. 7,234,321. In the process taught in US 2014/0083132, however, a closed-loop mixed-refrigerant circuit is not used, the natural gas feed stream instead being pre-cooled using an open-loop gaseous methane expander cycle and the methane vapor stream that is separated from the natural gas feed stream after partial liquefaction of the natural gas feed stream in the liquefying expander.
U.S. Pat. No. 4,778,497 teaches a process for producing a liquid cryogen in which a feed gas (the cryogen) is liquefied using an open-loop gaseous expander cycle that uses the feed gas as the refrigerant. The liquefied cryogen is then sub-cooled in a sub-cooler heat exchanger that uses a flashed portion of the end-product as refrigerant. Exemplary feed gases that can be liquefied using the process include helium, hydrogen, atmospheric gases, hydrocarbon gases, and mixtures of the aforementioned gases, such as air or natural gas.
U.S. Pat. No. 3,616,652 teaches a process for liquefying natural gas in which an open-loop gaseous expander cycle is used to liquefy the natural gas. The liquefied natural gas is then flashed and separated to provide the LNG product and a flash gas that is used as the refrigerant in the gaseous expander cycle.